A clean-burning alternative fuel for internal combustion engines is provided by the vaporization of high purity liquid methane. The operation of internal combustion engines on vaporized liquid methane fuel requires that the methane be of high purity, typically containing at least 98 mole % methane and preferably less than 0.5 mole % of hydrocarbons heavier than methane. High purity liquid methane can be produced from natural gas by known methods for separating methane from the heavier hydrocarbons which are present in natural gas. Technology for the storage and vaporization of liquid methane is known for both vehicular and stationary applications.
Natural gas can be processed by cryogenic distillation to remove hydrocarbon components heavier than methane and non-hydrocarbon components lighter than methane. This well-known technology is described in representative U.S. Pat. Nos. 3,837,172, 4,857,078, 5,114,451, 5,359,856, 5,390,499, 5,600,969, and 5,615,561. Combined cryogenic and non-cryogenic processes also have been used to produce high purity methane as described for example in U.S. Pat. No. 5,414,190.
Refluxing heat exchangers, also known as dephlegmators, have been used widely in combination with cryogenic distillation to separate hydrocarbon-containing mixtures. Dephlegmator technology is discussed in the review article entitled "Recovery of Valuable Hydrocarbons Using Dephlegmator Technology" by D. P. Bernhard and H. C. Rowles in Advances in Cryogenic Engineering, Vol 33, 1988, pp. 983-989. Representative processes using dephlegmators for the separation of hydrocarbon mixtures, some of which contain hydrogen and/or nitrogen, are described in U.S. Pat. Nos. 4,270,939, 4,270,940, 4,622,053, 4,714,487, 4,749,393, 4,720,293, 4,732,598, and 4,921,514.
Dephlegmators offer simple, reliable, and efficient operation for a wide variety of gas separations. They are particularly useful in the separation of mixtures of lower molecular weight components, which separation generally requires significant amounts of low temperature refrigeration. The invention described in the present specification and defined by the claims which follow is an efficient process utilizing dephlegmators for the production of high purity liquid methane.